This invention relates to voltage regulators. More particularly, this invention relates to circuits and methods for reducing the quiescent current in switching voltage regulators.
The purpose of a voltage regulator is to provide a predetermined and substantially constant output voltage to a load from a poorly-specified and fluctuating voltage source. One type of voltage regulator commonly used to accomplish this task is a switching voltage regulator. Switching voltage regulators are typically arranged to have a switching element (e.g., a power transistor) and an inductor coupled between the voltage source and the load. The switching regulator regulates the voltage across the load by turning the switching element ON and OFF so that power is transmitted through the switching element and into the inductor in the form of discrete current pulses. The inductor and an output capacitor then convert these current pulses into a steady load current so that the load voltage is regulated.
To generate a stream of current pulses, switching regulators include control circuitry that commands the switching element ON and OFF. The duty cycle of the switching element (i.e., the amount of time the switching element is ON compared to the period of an ON/OFF cycle), which controls the flow of current into the load, can be varied by a variety of methods. For example, the duty cycle can be varied by fixing the pulse stream frequency and varying the ON or OFF time of each current pulse, or by fixing the ON or OFF time of each current pulse and varying the pulse stream frequency.
Because switching regulators can operate at high levels of efficiency, they are often used in battery operated systems such as notebook computers, cellular telephones, and hand-held instruments. In such systems, when the regulator is supplying close to its rated output current, the efficiency of the overall circuit is usually high. However, this efficiency is generally a function of output current and typically decreases when the switching regulator is providing small amounts of current. This reduction in efficiency is generally attributable to the losses associated with operating the switching regulator. These losses include, among others, quiescent current losses, losses in the control circuit of the switching regulator, switching element losses, switching element driver losses, and inductor/transformer winding and core losses.
The reduction in efficiency of switching regulators at low output currents is of concern to circuit designers. This is because it is common for battery operated devices to experience short periods of high power use (i.e., periods during which relatively large currents must be supplied to a load), followed by extended periods of low power use (i.e., "standby" time during which a very small load current flows, but a regulated output voltage must be maintained). If the standby periods far exceed the usage periods, the quiescent current (i.e., the input current that flows into the switching regulator when the output is unloaded but still in voltage regulation) will determine the effective life of the battery. Accordingly, it is desirable to reduce quiescent current consumption as much as possible to extend battery life.
In the past, numerous techniques have been employed to reduce quiescent current losses in switching regulators during standby periods. For example, a switching regulator such as the LT1070 from Linear Technology Corporation, Milpitas, Calif., uses a control circuit that includes a comparator circuit and an error amplifier for monitoring the regulated output signal. When the output of the error amplifier drops below a threshold voltage, the regulator shuts down some of its internal circuitry to reduce quiescent current levels.
Other switching regulators from Linear Technology Corporation, such as the LT1307, LT1500, and LTC1625 use a mode of operation called "Burst Mode.TM." to reduce quiescent current. This mode of operation recognizes that the efficiency of a typical switching regulator drops off as the load decreases, because a fixed amount of power is wasted in the switch drive circuitry that is independent of load size. These switching regulators reduce quiescent current by holding the switching transistor(s) OFF, and turns OFF unneeded internal circuits, when the load current drops below a certain value.
A typical prior art current-mode stepdown switching regulator 100 employing burst mode operation is shown in FIG. 1. Voltage regulator 100 generally comprises an output circuit 110, a control circuit 130, and a filter circuit 125.
The voltage regulator of FIG. 1 operates as follows. A switch timing circuit 101 (which may be, for example, a one-shot, an oscillator, or any other suitable circuit) within control circuit 130 supplies a control signal SW ON that sets a latch 104. While latch 104 is set, a switch driver 106 provides a signal to output circuit 110 that causes a switch 108 in output circuit 110 to turn ON and provide current from an input voltage source VIN to an output node 117. Latch 104 remains set until an output signal from a current comparator 102 causes latch 104 to reset. When reset, latch 104 turns switch 108 OFF so that current is no longer drawn from VIN. Current comparator 102 determines when to reset latch 104 by comparing a current signal (I.sub.L) from output circuit 110 with a current threshold value (I.sub.TH) generated by an error amplifier 122 in control circuit 130 (discussed in more detail below).
The primary purpose of output circuit 110 is to provide current pulses as directed by control circuit 130 and to convert those current pulses into a substantially constant output current. Output circuit 110 includes power switch 108 coupled to VIN and a node 109, a catch diode 112 coupled from node 109 to ground, an inductor 114 coupled from node 109 to output node 117, a capacitor 116 coupled from output node 117 to ground, and a voltage divider formed by resistors 118 and 120 coupled from node 117 to ground. Although switching element 108 is depicted as a field-effect transistor (FET) in FIGS. 1 and 2, any other suitable switching element may be used if desired.
The operation of output circuit 110 can be divided into two periods. The first is when power switch 108 is ON, and the second is when power switch 108 is OFF. During the ON period, current passes from VIN through switch 108 and flows through inductor 114 to output node 117. During this period diode 112 is reverse-biased. However, after power switch 108 turns OFF, inductor 114 still has current flowing through it. The former current path from VIN through switch 108 is now open-circuited, causing the voltage at node 109 to drop such that catch diode 112 becomes forward-biased and starts to conduct. This maintains a closed current loop through a load (not shown). When power switch 108 turns ON again, the voltage at node 109 rises such that diode 112 becomes reverse-biased and turns OFF.
As shown in FIG. 1, error amplifier 122 in control circuit 130 senses the output voltage of regulator 100 via a feedback signal V.sub.FB produced by resistors 118 and 120. Error amplifier 122, which is preferably a transconductance amplifier, compares V.sub.FB with a reference voltage (V.sub.REF) that is also connected to amplifier 122. An output signal, I.sub.TH, is generated in response to this comparison. The I.sub.TH signal is filtered by a filter circuit 125 comprised of resistor 124 and capacitor 126 and coupled to an input of current comparator 102. The value of I.sub.TH sets the point at which current comparator 102 trips.
An input of a burst comparator 128 in control circuit 130 is also coupled to the output of error amplifier 122 and receives the filtered I.sub.TH signal. Burst comparator 128 monitors I.sub.TH as an indication of load current and compares the filtered I.sub.TH signal with a voltage potential V1 that is connected to another input burst comparator 128. V1 is typically set to a value that represents the minimum current value for which it is desirable to maintain regulator 100 in the normal operating mode. This is usually a fraction of the maximum rated output current for regulator 100. When I.sub.TH decreases to or below the value of V1, burst comparator 128 trips, sending a SLEEP signal to the shutdown enable inputs of current comparator 102 and switch timing circuit 101. This shuts down current comparator 102 and switch timing circuit 101, maintains switch 108 OFF, and thus places voltage regulator 100 in a standby mode so that quiescent current is reduced. While in standby mode, capacitor 116 supports the load and no switching losses are incurred. When the output voltage V.sub.OUT has decayed slightly, causing the I.sub.TH voltage to increase by the amount of the hysteresis in burst comparator 128, the SLEEP output of comparator 128 is de-asserted and normal operation resumes.
When a large load step is applied to voltage regulator 100, the current drawn from regulator 100 increases. This causes a slight reduction in the value of V.sub.FB which, in turn, causes I.sub.TH to increase. The increase in I.sub.TH raises the threshold point at which current comparator 102 trips, resulting in an increase in the current supplied by the regulator to match the required load current.
However, when the demand for load current decreases, switching element 108 will continue to turn ON each cycle as directed by control circuit 130, but the value of I.sub.TH will decrease in order to turn switching element 108 OFF at lower currents. Switching element 108 therefore continues to switch at the same frequency, but it conducts less current as the load current decreases, causing switching losses to become a larger percentage of the output power.
As the load current decreases further, I.sub.TH periodically drops below V1 and voltage regulator 100 begins to experience standby periods. As the demand for load current drops even further, the standby periods become longer. Finally, voltage regulator 100 enters a prolonged standby state in which substantially no current is supplied to the load. When this occurs, the operating intervals become so infrequent that the input current to voltage regulator 100 is essentially defined by the quiescent current alone.
One significant limitation in the amount of quiescent current reduction possible in voltage regulator 100 is the need of error amplifier 122 to quickly slew filter circuit 125 when transitioning from standby mode to normal operating mode. Such a transition is necessary, for example, when a large load step is placed on voltage regulator 100 during a standby period. If the output current of error amplifier 122 cannot slew filter circuit 125 as fast as the load current is slewing output capacitor 116, the recovery time from standby periods will be extended. This undesirably causes the output voltage (V.sub.OUT) to undershoot.
In addition, because the value of capacitor 126 is typically large enough such that filter circuit 125 provides adequate filtering, slewing filter circuit 125 requires a relatively large output current capability from error amplifier 122. Accordingly, amplifier 122 must constantly be able to supply this current so that regulator 100 can quickly respond to large load steps, even when it is in a standby state. As a result, amplifier 122 must undesirably draw substantial amounts of quiescent current even while regulator 100 is in a standby period.
Other prior art voltage regulators which operate at low quiescent currents are also presently available. For example, the LT1316 and LTC1474 from Linear Technology Corporation are able to operate at extremely low quiescent currents while in standby mode by replacing the above-described error amplifier with a micropower comparator that uses a voltage reference as a fixed comparator threshold. This type of regulator determines when to enter and exit standby periods by comparing the load voltage (which is indicative of load current) with the fixed voltage reference. Thus, during standby periods, only the fixed voltage reference and a micropower comparator circuit are active, which significantly reduces the quiescent current required by the regulator. However, because this type of regulator usually employs a fixed current threshold, it cannot adaptively increase the threshold value with increasing load current, which may limit the regulator's response to large current demands.
In view of the foregoing, it would be desirable to provide a circuit and method for operating switching mode voltage regulators at very low quiescent current levels during standby periods and yet provide large output current during periods of normal operation.
It would also be desirable to provide a circuit and method for allowing a fast transition in switching regulators from a low-output current, very low-power consumption state (standby or burst mode) to a high-current output state by optimizing the response time of control circuitry in the switching regulators during the transition period.